In a molded-case circuit breaker (MCCB), an arc column is not always kept to burn inside deion plates but several times is shorten in the narrow area between electrodes. This unstable arc behavior causes several sudden drops in the arc voltage and suspension of the current limiting process. In this paper, an air-buffer chamber is proposed to be mounted in a MCCB to control the unstable arc motion and its effect on the current limiting performance of the MCCB is experimentally discussed.
It is found from the measurement of the attenuation of the projected 70GHz microwave that the temperature of the narrow area between electrodes begins to rise before the sudden drops in the arc voltage. This suggests us that the unstable arc motion is brought about with the high temperature air flow. Then, an air buffer chamber is mounted behind the arc chamber to efficiently exhaust the high temperature air as well as to hold the arc column inside the deion plates. The arcing time is reduced to 80% of that in the normal MCCB by mounting the air buffer cylinder of 20mm dia., 70mm length.